Heat shock genes (hsp genes) are known in a variety of organisms, including yeast, Drosophila, and some plant species. One class of heat shock genes expresses protein in response to heat stress only. Another class of heat shock genes has a low basal level of activity that is highly elevated upon heat induction.
When genetically engineering a heterologous gene in plants, the selection of a promoter is often a critical factor. While it may be desirable to express certain genes only in response to a particular stimulus, or localize their expression in certain tissues, other genes are more desirably expressed constitutively, i.e., throughout the plant at all times and in most tissues. In the past, the 35S promoter from Cauliflower Mosaic Virus (CaMV) has been used for constitutive expression of heterologous genes. For regulatory and other reasons it would be desirable to regulate heterologous gene expression with a promoter which is not of pathogenic origin. In addition, use of a plant promoter may alter the level of activity in particular tissues and may alter the spectrum of tissues in which expression is achieved in comparison with viral promoters.